Transition metal doped spinel type mgal2o4 fluorescent material and laser apparatus using, and method of making, such fluorescent material

ABSTRACT

A transition metal doped spinel type MgAl 2 O 4  fluorescent material capable of laser oscillation and a laser apparatus using the same. It is made from a mixed raw material of an Al raw material and a Mg raw material added thereto in an amount together with a transition metal raw material so that the amount of Mg exceeds that of Al by a few percents in terms of molar ratio, by shaping the mixed raw material under pressure to form a source material rod, and single-crystallizing the source material rod in a selected gaseous atmosphere by floating zone melting. The doping amount of Ti is such that in composition formula MgAl 2-x Ti x O 4  it lies in a range: 0.003≦x≦0.01. The doping amount of Mn is such that in composition formula Mg 1-x Mn x AlO 4  it lies in a range: 0.003≦x≦0.01.

TECHNICAL FIELD

The present invention relates to a transition metal doped spinel typeMgAl₂O₄ fluorescent material, i.e., a fluorescent material having acrystalline matrix made of MgAl₂O₄ single crystal with a spinelcrystallographic structure and doped with a transition metal. It alsorelates to a transition metal doped MgAl₂O₄ laser apparatus, i.e., alaser apparatus using such a fluorescent material. The invention furtherrelates to a method of making a transition metal-doped MgAl₂O₄fluorescent material.

BACKGROUND ART

Fluorescent materials are known to be indispensable to a variety ofcolor displays. Among them, there are also some classes of fluorescentmaterials doped with emission centers in an insulator crystallinematrix. Because of a broad emission spectrum, such a fluorescentmaterial when used as a laser medium for a laser apparatus of externalresonator type, is capable of laser oscillations over variouswavelengths and when used as a laser medium for a source of anultrashort light pulse, it is possible to make its time width extremelyshort. And, because of the facts that its fluorescent emissionefficiency is high and that the higher of melting point, its matrixcrystal the higher is the output of a laser using it, active researcheson fluorescent materials of this type whose melting point of matrixcrystals are high, have been conducted.

For example, while a ruby laser and a Ti-doped sapphire laser, both ofwhich have a corundum type crystallographic structure, are widelyutilized as excellent lasers, their laser medium is a fluorescentmaterial of which the matrix crystal is of Al₂O₃ and the emissioncenters are of a transition metal. Excellent properties of these lasersowe much to the high melting point of their matrix crystal (i.e., amelting point of 2050° C.) as well as to its high crystal perfectionwhich allows it to form an energy level that is high in terms ofemission efficiency and optimum for laser oscillations when doped with atransition metal. Thus, if a fluorescent material can be made having aMgAl₂O₄ matrix crystal of spinel type crystallographic structure thathas a melting point (i.e., 2135° C.) higher than that of the Al₂O₃crystal and that is yet excellent in crystal perfection, it would makeit possible to provide a laser that is yet higher in output.

However, although a fluorescent material having a MgAl₂O₄ matrix crystalwith a spinel type crystallographic structure has been studied andexpected as a fluorescent material that excels ruby and Ti-dopedsapphire lasers, no such fluorescent material capable of laseroscillation has up to date been successfully obtained.

Document 1 (L. E. Bausa, I. Vergara and J. Garcia-sole: J. Appl. Phys.68 (2), 15 Jul. 1990, p. 736) describes a Ti doped spinel type MgAl₂O₄fluorescent material having Ti doped at 0.05 atomic %, made by theVerneuil method. As seen from FIG. 1 in Document 1, the fluorescentmaterial has absorption peaks at 490 nm and 790 nm, besides band edgeabsorption. Also, as shown in FIG. 2 of the Document, the fluorescentmaterial has an emission peak of 465 nm by band edge excitation (266 nm)and an emission peak of 805 nm by excitation at 532 nm. Further, the 490nm absorption, the 465 nm emission and the 805 emission of thefluorescent material are according to d-d transitions of d—electrons ofTi³⁺ located at B site of the spinel crystal in ligand field of 6oxygens. Also, the 790 nm absorption is regarded as by Ti⁴⁺—Fe²⁺ pair.Thus, the absorptions and emissions of this fluorescent material aremainly caused by d-d transitions most of which are electric dipoleforbidden transitions and its emission efficiency must therefore be low.Further, there are absorptions by impurities observed, too, so the laseroscillation with this crystal is thought to be impossible. While thedocument states that using the 805 nm emission peak will make itsinfrared laser oscillation possible, this has not been confirmed.

Document 2 (R. Clausen and K. Peterman: IEEE Journal Q.E. E. 24 (1988)1114) and Document 3 (K. Peterman et al., Opt. Commun. 70 (1989) 483)describe Mn doped spinel type MgAl₂O₄ fluorescent materials having Mndoped by 1 atomic % and 18 atomic %, made by the Czochralski andVerneuil methods. As seen from FIG. 1 in Document 3, these fluorescentmaterials have many absorption peaks over a wavelength range from 250 nmto 550 nm. These absorption peaks are according to d-d transitions ofd—electrons of Mn²⁺ located at A site of the spinel crystal in ligandfiled of 4 oxygens. Since most of d-d transitions are electric dipoleforbidden transitions, its emission efficiency is considered to be lowand its laser oscillation to be impossible. Indeed, the document statesthat as an electron excited to a first excitation state of the d-dtransitions in the fluorescent material is further excited to an upperlevel, its laser oscillation is not possible.

Thus, while spinel type MgAl₂O₄ fluorescent materials doped with varioustransition metals have hitherto been tried, no such fluorescent materialcapable of laser oscillation has been brought to realization.

In view of the problems mentioned above, the first object of the presentinvention is to provide a transition metal doped spinel type MgAl₂O₄fluorescent material that is capable of laser oscillation. Also, asecond object of the present invention is to provide a transition metaldoped MgAl₂O₄ laser apparatus, i.e., a laser apparatus using such afluorescent material. Further, a third object of the present inventionis to provide a method of making a transition metal doped MgAl₂O₄fluorescent material.

DISCLOSURE OF THE INVENTION

In order to achieve the first object mentioned above, a transition metaldoped spinel type MgAl₂O₄ fluorescent material in accordance with thepresent invention is characterized in that it is made by growing asingle-crystal from a source material rod in a selected gaseousatmosphere wherein the source material rod is formed from a mixed rawmaterial of an Al raw material and a Mg raw material added thereto in anamount together with a transition metal raw material so that the amountof Mg exceeds that of Al by a few percents in terms of molar ratio. Thesaid selected gaseous atmosphere may be an oxidizing or inert gas, andthe said source material rod can be single-crystallized, preferablyusing floating zone melting.

The fluorescent material according to the present invention isessentially free from vacancies of Mg or oxygen, and indeed the materialundoped with a transition metal, namely the spinel type MgAl₂O₄ matrixcrystal itself is high in crystalline perfection to an extent that ithas no absorption peak for light of a wavelength ranging from 300 nm to900 nm.

The fluorescent material according to the present invention may becharacterized in that it uses the above-mentioned matrix crystal, andthe said transition metal is Ti and the said fluorescent material has acomposition expressed by chemical formula MgAl_(2-x)Ti_(x)O₄ where0.003≦x≦0.01 and that it has no absorption peak except for its band edgeabsorption for light wavelengths of 200 nm to 900 nm and it is capableof light emission having a peak at 490 nm by its band edge excitation.

In the Ti doped spinel type MgAl₂O₄ fluorescent material describedabove, Ti may exist as Ti⁴⁺ at a B site in the spinel type MgAl₂O₄crystal, and the light emission may occur when the electron of anelectron-hole pair generated by band edge excitation is captured by aTi⁴⁺ to form an intermediate energy state and recombines with a holecaptured by an O in the vicinity of the Ti⁴⁺. The light emission mayalso occur conversely when a hole is captured by an O in the vicinity ofa Ti⁴⁺ to form an intermediate energy state and recombines with anelectron in the vicinity of the Ti⁴⁺. Since this emission process is notby electric dipole forbidden transition but by charge transfertransition which is one or two order higher in transition probabilitythan the electric dipole forbidden transition in the d-d transition, afluorescent material provided here is having a high emission efficiencyrendering it capable of laser oscillation.

The fluorescent material according to the present invention may also becharacterized in that it uses the above-mentioned matrix crystal, andthe said transition metal is Mn and the said fluorescent material has acomposition expressed by chemical formula Mg_(1-x)Mn_(x)Al₂O₄ where0.003≦x≦0.01 and in a wavelength range of 200 nm to 900 nm it has itsband edge absorption and absorption with a peak at 450 nm of whichintensity increases as the amount of doped Mn is increased, and it haslight emission having a peak at 520 nm by its band edge excitation orexcitation at 450 nm and light emission having a peak at 650 nm by itsband edge excitation.

In the Mn doped spinel type MgAl₂O₄ fluorescent material describedabove, the emission at 650 nm is caused by Mn existing as Mn²⁺ at an Asite in the spinel type MgAl₂O₄ crystal. The electron of anelectron-hole pair generated by band edge excitation is captured by aMn²⁺ to form an intermediate energy state and recombines with a holecaptured by an O in the vicinity of the Mn²⁺. The emission may alsooccur conversely so that a hole is captured by an O in the vicinity of aMn²⁺ to form an intermediate energy state and recombines with anelectron in the vicinity of the Mn²⁺. Since this emission process is notby electric dipole forbidden transition but by charge transfertransition which is one or two order higher in transition probabilitythan the electric dipole forbidden transition in the d-d transition andalso since the emission at 520 nm is caused by both the d-d and chargetransfer transitions, the fluorescent material provided here has highemission efficiency rendering it capable of laser oscillation.

The fluorescent material according to the present invention may also becharacterized in that the said transition metal doped in the matrixcrystal, is V and the said fluorescent material has a compositionexpressed by chemical formula MgAl_(2-x)V_(x)O₄ where 0.001≦x≦0.01 andthat it has no absorption peak except for its band edge absorption for awavelength range of 200 nm to 900 nm and it is capable of white colorlight emission over wavelengths of 450 nm to 750 nm by its band edgeexcitation. So composed as described, the V doped spinel type MgAl₂O₄fluorescent material is a fluorescent material which like the Ti and Mndoped fluorescent materials described above, is high in emissionefficiency so as to be capable of laser oscillation.

In order to achieve the second object mentioned above, there is alsoprovided in accordance with the present invention a transition metaldoped spinel type MgAl₂O₄ laser apparatus characterized in that it has alaser medium made of a transition metal doped spinel type MgAl₂O₄fluorescent material discussed above.

The laser apparatus according to the present invention may becharacterized in that the laser medium is made of a said Ti doped spineltype MgAl₂O₄ fluorescent material and that the said laser medium isside-pumped with a fourth harmonic of a Nd: YAG laser (having awavelength of 266 nm), a fourth harmonic of a Nd: YLF (having awavelength of 262 nm) or a fourth harmonic of a Nd: YAP laser (having awavelength of 269 nm) and is caused to resonate with an external laserresonator to produce blue or green color laser oscillation lightutilizing light emission having the peak at 490 nm of the saidfluorescent material.

The laser apparatus according to the present invention may also becharacterized in that the said laser medium is a said Mn doped spineltype MgAl₂O₄ fluorescent material and that the said laser medium isside-pumped with a fourth harmonic of a Nd: YAG laser (having awavelength of 266 nm), a fourth harmonic of a Nd: YLF (having awavelength of 262 nm) or a fourth harmonic of a Nd: YAP laser (having awavelength of 269 nm) and is caused to resonate with an external laserresonator to produce blue or green and/or red color laser oscillationutilizing light emission having the peak or peaks at 520 nm and/or 650nm of the said fluorescent material.

The laser apparatus according to the present invention may also becharacterized in that two Ti doped spinel type MgAl₂O₄ fluorescentmaterials and a Mn doped spinel type MgAl₂O₄ fluorescent material aredisposed in series in an external laser resonator and these threefluorescent materials are side-pumped with a fourth harmonic of a Nd:YAG laser (having a wavelength of 266 nm), a fourth harmonic of a Nd:YLF (having a wavelength of 262 nm) or a fourth harmonic of a Nd: YAPlaser (having a wavelength of 269 nm) to produce blue and green colorlaser oscillations, respectively, from the two Ti doped spinel typeMgAl₂O₄ fluorescent materials and red color laser oscillation from theMn doped spinel type MgAl₂O₄ fluorescent material, simultaneously. Thissetup provides a laser with the three primary colors.

The laser apparatus according to the present invention may also becharacterized in that said Ti doped spinel type MgAl₂O₄ fluorescentmaterial and two said Mn doped spinel type MgAl₂O₄ fluorescent materialsare disposed in series in an external laser resonator and these threefluorescent materials are side-pumped with a fourth harmonic of a Nd:YAG laser (having a wavelength of 266 nm), a fourth harmonic of a Nd:YLF (having a wavelength of 262 nm) or a fourth harmonic of a Nd: YAPlaser (having a wavelength of 269 nm) to produce blue or green colorlaser oscillation from the Ti doped spinel type MgAl₂O₄ fluorescentmaterial and blue or green and red color laser oscillations,respectively, from the Mn doped spinel type MgAl₂O₄ fluorescentmaterials, simultaneously. This setup provides a laser with the threeprimary colors, too.

The laser apparatus according to the present invention may also becharacterized in that said Ti doped spinel type MgAl₂O₄ fluorescentmaterial and a Mn doped spinel type MgAl₂O₄ fluorescent material aredisposed in series in an external laser resonator and these twofluorescent materials are side-pumped with a fourth harmonic of a Nd:YAG laser (having a wavelength of 266 nm), a fourth harmonic of a Nd:YLF (having a wavelength of 262 nm) or a fourth harmonic of a Nd: YAPlaser (having a wavelength of 269 nm) to produce blue or green colorlaser oscillation from the said Ti doped spinel type MgAl₂O₄ fluorescentmaterial and blue or green and red color laser oscillations from thesaid Mn doped spinel type MgAl₂O₄ fluorescent material, simultaneously.This setup provides a laser with the three primary colors, too.

In order to achieve the third object mentioned above there is providedin accordance with the present invention, a method of making atransition metal doped spinel type MgAl₂O₄ fluorescent material,characterized in that it comprises: preparing a mixed raw material of anAl raw material and a Mg raw material added thereto in an amounttogether with a transition metal raw material so that the amount of Mgexceeds that of Al by a few percents in terms of molar ratio, shapingthe mixed raw material under pressure to form a source material rod, andgrowing a single-crystal from the source material rod in a selectedgaseous atmosphere by floating zone melting method. The said selectedgaseous atmosphere may be an oxidizing or inert gas.

Adopting this method, it is possible to obtain a spinel type MgAl₂O₄matrix crystal that is essentially free from vacancies of Mg or oxygenand is high in crystalline perfection to an extent, for example, thatthe material without a transition metal, namely the matrix crystal hasno absorption peak for light wavelengths of 300 nm to 900 nm.

If the said Al raw material is Al₂O₃, the said Mg raw material is MgOand the said transition metal raw material is TiO₂ and the raw materialTiO₂ is added to the raw material Al₂O₃ at a proportion in a range of0.003 to 0.01 in terms of molar ratio, it is then possible to make a Tidoped spinel type MgAl₂O₄ fluorescent material that has no absorptionpeak except for its band edge absorption for light wavelength range from200 nm to 900 nm and is capable of light emission having a peak at 490nm by its band edge excitation.

If the said Al raw material is Al₂O₃, the said Mg raw material is MgOand the said transition metal raw material is MnO₂ and the raw materialMnO₂ is added to the raw material MgO at a proportion in a range of0.003 to 0.01 in terms of molar ratio, it is then possible to make a Mndoped spinel type MgAl₂O₄ fluorescent material which besides its bandedge absorption, has an absorption peak at 450 nm of which intensityincreases proportionally as the amount of doped Mn is increased andwhich is capable of light emission having a peak at 520 nm by excitationlight at 450 nm while having a peak at 650 nm by its band edgeexcitation.

Also, if the said Al raw material is Al₂O₃, the said Mg raw material isMgO and the said transition metal raw material is a V metal and the Vmetal raw material is added to the raw material Al₂O₃ at a proportion ina range of 0.001 to 0.01 in terms of molar ratio, it is then possible tomake a V doped spinel type MgAl₂O₄ fluorescent material that has noabsorption peak except for its band edge absorption for lightwavelengths of 200 nm to 900 nm and is capable of white color lightemission ranging over wavelengths from 450 nm to 750 nm by its band edgeexcitation.

Further, a transition metal doped spinel type MgAl₂O₄ fluorescentmaterial may also be made by mixing together a source material Al₂O₃, asource material MgO and a transition metal raw material to form amixture thereof; shaping under pressure, and sintering the mixture toform a sintered body; and growing epitaxially on a single-crystalsubstrate a single-crystal thin film of transition metal doped spineltype MgAl₂O₄ fluorescent material by laser ablation using the saidsintered body as a target in an O₂ gas.

BRIEF DESCRIPTION OF THE DRAWINGS

In the drawings:

FIG. 1 is a conceptual view illustrating a diagrammatic cross section ofa hydrostatic forming apparatus for use in making a transition metaldoped spinel type MgAl₂O₄ fluorescent material according to the presentinvention;

FIG. 2 is a diagram conceptually illustrating a floating zone meltingfurnace for use in the making according to the present invention;

FIG. 3 is a view illustrating a diagrammatic cross section of a die setfor use in the making according to the present invention;

FIG. 4 is a diagrammatic cross sectional view illustrating an electricfurnace heating system that is a sintering apparatus for use in themaking according to the present invention;

FIG. 5 is a conceptual view of a laser ablation apparatus for use in themaking according to the present invention;

FIG. 6 is a graph showing emission spectra of light emissions by bandedge excitation of matrix crystals according to the present invention;

FIG. 7 is a graph showing transmittance characteristics of a matrixcrystal and Ti doped fluorescent materials according to the presentinvention;

FIG. 8 is a graph showing emission spectra of light emissions by bandedge excitation light of Ti doped fluorescent materials according to thepresent invention;

FIG. 9 is graph showing time resolved emission spectra of a lightemission spectrum having a peak at 490 nm of a Ti doped fluorescentmaterials according to the present invention;

FIG. 10 is a graph showing transmittance characteristics of Mn dopedfluorescent materials according to the present invention;

FIG. 11 is a graph showing emission spectra of light emissions by 450 nmexcitation light of Mn doped fluorescent materials according to thepresent invention;

FIG. 12 is a graph showing excitation spectra of emission spectra probedat a peak of 520 nm of Mn doped fluorescent materials according to thepresent invention;

FIG. 13 is a graph showing time resolved spectra of an emission spectrumhaving a peak at 520 nm of a Mn doped fluorescent material according tothe present invention;

FIG. 14 is a graph showing emission spectra of light emissions by bandedge excitation of Mn doped fluorescent materials according to thepresent invention;

FIG. 15 is a graph showing time resolved spectra of an emission spectrumhaving a peak at 650 nm of a Mn doped fluorescent material according tothe present invention;

FIG. 16 is a graph showing emission spectra of light emissions by bandedge excitation of V doped fluorescent materials according to thepresent invention;

FIG. 17 is a picture showing a RHEED (reflection high-energy electrondiffraction) pattern of a thin film single crystal fluorescent materialof MgAl_(2-x)Ti_(x)O₄ grown epitaxially on a SrTiO₃ (100) substrate.

FIG. 18 is a chart showing an X-ray diffraction pattern of the thin filmsingle crystal fluorescent material of MgAl_(2-x)Ti_(x)O₄;

FIG. 19 is a photograph showing fluorescence emitted from thin filmsingle crystal fluorescent material of MgAl_(2-x)Ti_(x)O₄ when it isexcited by an electron beam;

FIG. 20 shows embodiments in makeup of a transition metal doped spineltype fluorescent material laser apparatus according to the presentinvention;

FIG. 21 is a photograph showing laser oscillations of a Ti doped spineltype fluorescent material laser apparatus of the present invention; and

FIG. 22 shows laser oscillations of a Mn doped spinel type fluorescentmaterial laser apparatus of the present invention.

BEST MODES FOR CARRYING OUT THE INVENTION

The present invention will better be understood from the followingdetailed description and the drawings attached hereto showing certainillustrative forms of implementation of the present invention.Therefore, it should be noted that such forms of implementationillustrated in the accompanying drawings hereof are intended in no wayto limit the present invention but to facilitate an explanation andunderstanding thereof.

Mention is first made of a method of making a fluorescent materialaccording to the present invention.

A method of making a fluorescent material in accordance with the presentinvention comprises the step of mixing an aluminum oxide raw material, amagnesium oxide raw material and a transition metal raw materialtogether, the step of shaping the mixture under pressure to form asource material rod, and a melt growth step of placing the sourcematerial rod on a seed rod of the source material in a floating zonemelting furnace and growing single-crystal by a floating zone meltingmethod in a gaseous atmosphere.

Mention is now made of a pressure forming apparatus for use in themaking method of the present invention.

FIG. 1 is a conceptual view illustrating a diagrammatic cross section ofa pseudo-hydrostatic forming apparatus. In FIG. 1, thepseudo-hydrostatic forming apparatus, designated by reference character10, comprises a cylinder 11 filled with water 15, a piston 13 and an oilhydraulic press (not shown). And, a source material mixture 17 formedinto a columnar shape, e.g., by being packed into a tube type balloon(with a diameter of about 6 mm and a length of about 250 mm), is placedin the water 15 and compressed and shaped when the piston 13 is pushedby the oil hydraulic press. This process gives a source material rod ofa suitable length shown as the mixture 17. In this case, the oilhydraulic press exerts a load of, e.g., 300 kg/cm², and the sourcematerial rod has a length of, e.g., 50 to 100 mm while a seed rod is ofa length of, e.g., 20 to 50 mm.

Mention is next made of a floating zone melting furnace for use inmaking of a fluorescent material of the present invention. FIG. 2 is aconceptual view illustrating such a floating zone melting furnace. InFIG. 2, the floating zone melting furnace, designated by referencecharacter 20, comprises a fused silica tube 23 in which to maintain agaseous atmosphere therein, argon or oxygen gas or both can beintroduced from a gas supply system such as a cylinder (not shown). Italso includes a pair of shafts 21 and 21 which hold between them a seedrod 22 lying lower and a source material rod 26 lying upper in the fusedsilica tube 23 and which are jointly rotatable and movable vertically.Included further are a spheroidal bifocal mirror 27, an infraredconvergent heating source 25 such as a xenon lamp disposed at one focalposition of the bifocal mirror 27, and a window for observation (notshown). And, the floating zone melting furnace 20 is associated with acomputer (not shown) for controlling the rotary and vertical movementsof the shafts 21 and 21 and their heating temperature and rates oftemperature rise and fall.

Further, an end of the seed rod 22 held by one of the shafts 21 and anend of the source material rod 26 held by the other shaft 21 arepositioned to lie at the other or second focal position of thespheroidal mirror 27 so that by controlling vertical movement of theshafts 21 and 21 allows a desired growth. In FIG. 2, note also thatinfrared rays 29 are drawn in order to show the condensation of light atthe second focal position of the spheroidal mirror 27.

In the floating zone melting furnace 20 constructed as mentioned above,after heating and melting these ends of the seed rod 22 and sourcematerial rod 26, two ends are contacted to form a melt zone 24. Thus theshafts 21 and 21 are slowly moved downwards to move the melt zonegradually towards the source material rod to allow a single crystal tobe growing on the seed rod.

Thus, in the floating zone melting method in accordance with the presentinvention, a MgAl₂O₄ single crystal having a spinel typecrystallographic structure and doped with a transition metal at itsadequate positions in matrix crystal can be made in a short period oftime and moreover in a size of, e.g., a diameter of 5 mm and a length of100 mm or more. Furthermore, without the need to use a crucible to holda melt, it is possible to make such a single crystal that is free ofcontamination by impurities and that has a same concentration for thetransition metal as in the source material.

Mention is next made of a method and an apparatus for making a sinteredtarget for laser ablation that may be used in the making of afluorescent material according to the present invention.

The method of making such a sintered target comprises a mixing step ofmixing an aluminum oxide raw material, a magnesium oxide raw materialand a transition metal raw material together, a compression molding stepof forming the mixed source material to prepare a target shaped body anda sintering step of heating the formed body in a given gaseousatmosphere to make the sintered target.

Mention is next made of a pressure forming apparatus for compressionmolding that may be used in the making of a fluorescent materialaccording to the present invention. FIG. 3 is a conceptual crosssectional view illustrating such a die set. In the Figure, a mould ordie set 30 is shown comprising a metal cylinder 33 loaded with a mixtureor mixed source material 34, a pair of an upper and a lower pistons 31and 31 and a pressing unit (not shown) that applies pressure to thepistons 31 and 31 upward and downward. Pressing is cold pressing.

Mention is next made of a sintering apparatus for use in the making of afluorescent material according to the present invention.

FIG. 4 is a conceptual view diagrammatically illustrated cross sectionof an electric furnace heating system that constitutes the sinteringapparatus. In FIG. 4, the electric furnace heating system, designated byreference character 40, comprises an electric furnace of cylinder type41, an aluminum core tube 43 that withstands high temperature, a gassupply subsystem (not shown) for supplying a gas into the core tube 43and a vacuum pumping subsystem (not shown) that permits pumping up tohigh vacuum. The electric furnace heating system 40 also controls itsdegree of vacuum, heating temperature and rising and falling rates oftemperature. The vacuum pumping subsystem may use, for example, acombination of oil diffusion and rotary pumps.

In such an arrangement as shown in FIGS. 3 and 4, first a space formedof the pistons 31 and the metal cylinder 33 is filled with a sourcematerial mixture 34, and when a load, e.g., of 200 kg/cm², is applied tothe pistons 31 and 31 from both sides to compress and mold the sourcematerial mixture 34, a formed body 46 is produced, having a diameter of20 mm and a thickness of 5 mm, for example.

Next, the compression-molded formed body 46 is placed on an aluminaceramic boat 44 and then placed in the core tube 43. The core tube 43 isthen supplied with a selected gas while evacuated as indicated by thearrows in FIG. 4 while sintering proceeds. In this way, a sintered bodyis formed under a selected temporal profile of temperature in theelectric furnace 41.

A sintered body made in such a method is shaped as desired shape in thestage of pressure molding and thus can have a shape suitable for itsintended use.

Mention is next made of a laser ablation apparatus for use in the makingof a fluorescent material according to the present invention. FIG. 5 isa conceptual view of a laser ablation apparatus. The laser ablationapparatus, indicated by reference character 50, comprises a vacuumchamber 51, a substrate retaining and heating unit 53 that holds asubstrate 52 and controls its temperature in the vacuum chamber 51, anda gas supply unit 54 for introducing a gas into the vacuum chamber 51.It also includes a target 55 whose material is to be vapor-deposited onthe substrate 52 and a laser light source for laser ablation 57 disposedoutside of the vacuum chamber 51 for irradiating the target 55 with apulsed laser light 56 to vaporize the target material therefrom. Theapparatus 50 further includes a RHEED (reflection high-energy electrondiffraction) unit 58 made of an electron gun 58 a and an electron beamdetector 58 b for analyzing a structure of the material vapor-depositedon the substrate 52, which makes it possible to control the filmthickness of the vapor-deposited material (from the target 55) in anatomic layer unit. Using this unit allows obtaining a vapor-depositedthin film that has a same composition as that of the target material.

A thin film crystalline fluorescent material according to the presentinvention may be made by using this apparatus holding a single crystalsubstrate 52 at a selected temperature while laser-ablating materialfrom the target for vapor deposition on the substrate 52, the targetbeing a sintered body made by sintering a mixture of an aluminum oxidematerial, a magnesium oxide material and a transition metal materialwhich are put together in selected concentrations.

An explanation is next given of properties of a transition metal dopedspinel type MgAl₂O₄ fluorescent material by way of several specificexamples.

EXAMPLE 1

An aluminum oxide (Al₂O₃) material and a magnesium oxide (MgO) materialwere mixed together and a mixture thereof was compression-molded in apressure forming apparatus as shown in FIG. 1 to prepare a rod of sourcematerial, which was placed in a floating zone melting furnace as shownin FIG. 2 to single-crystallize the source material by the floating zonemelting method in gaseous atmosphere. A resultant MgAl₂O₄ matrix crystalwas irradiated with a ray of ultraviolet light having a peak at 360 nmand its emission spectrum was observed.

In this case, several specimens grown by floating zone melting methodwith varying the mixing ratio of the aluminum oxide (Al₂O₃) material andmagnesium oxide (MgO) material and with variously changing the gaseousatmosphere were prepared and their emission spectra were compared.

FIG. 6 is a graph comparing the emission spectra of MgAl₂O₄ matrixcrystals grown with the different conditions, when they were excitedwith a band edge excitation light of 275 nm. In the graph, the abscissaaxis represents the light wavelength for fluorescence and the ordinateaxis represents the emission intensity in an arbitrary scale. Curves (1)to (4) in the graph indicate the emission spectra of specimens whichwere grown by floating zone melting from a source material rod made ofthe aluminum oxide (Al₂O₃) and magnesium oxide (MgO) materials mixedtogether so that they are present in an equal amount in terms of molarratio, in (1) a gaseous atmosphere of mixed Ar and H₂ gases (with H₂ ata volume proportion of 2.4%), (2) an atmosphere of O²⁺ ion gas, (3) agaseous atmosphere of O₂ gas and (4) an atmosphere of Ar gas,respectively. Curve (5) indicates the emission spectrum of a specimenwhich was grown by floating zone melting from a source material rod madeof the aluminum oxide (Al₂O₃) and magnesium oxide (MgO) materials mixedtogether so that the amount of MgO exceeds that of Al₂O₃ by 1% in termsof molar ratio in an atmosphere of O₂ gas.

As is seen from the graph, from the specimens in which the aluminumoxide (Al₂O₃) and magnesium oxide (MgO) materials are mixed together inan equal amount in terms of molar ratio, light emissions are observedover the entire visible light range, regardless of the types of gaseousatmosphere. On the other hand, from the specimen (5) in which thealuminum oxide (Al₂O₃) and magnesium oxide (MgO) materials are mixedtogether so that the amount of MgO exceeds that of Al₂O₃ by 1% in termsof molar ratio, very weak light emission is observed over the entirevisible light range. Also although not shown, note here that from aspecimen prepared replacing the gaseous atmosphere with Ar gas, likewisevery weak light emission was observed over the entire visible lightrange.

It is seen, therefore, that an excellent optical crystal is obtained ifa mixed source material is made by mixing an Al raw material with a Mgraw material so that an amount of Mg exceeds that of Al in terms ofmolar ratio a few percents, by pressure-molding the mixed sourcematerial into a source material rod and by single-crystallizing thesource material rod by the floating zone melting method in an oxygen orAr gas.

EXAMPLE 2

Several source material rods were prepared from mixed source materialscontaining an aluminum oxide (Al₂O₃) material and a magnesium oxide(MgO) material in a proportion such that an amount of MgO exceeds thatof Al₂O₃ by 1% in terms of molar ratio, and containing with variousamounts of TiO₂ as the transition metal material added thereto, and thesource material rods were single-crystallized in an Ar or O₂ gas by thefloating zone melting method to form fluorescent materials, and theirproperties were measured.

FIG. 7 is a graph showing transmittance characteristics of a matrixcrystal and Ti doped fluorescent materials according to the presentinvention. In the graph, the abscissa and ordinate axes represent thelight wavelength and transmittance, respectively. The dashed dotted lineindicates the transmittance of the matrix crystal while the dotted line,the dashed line and the solid line indicate the transmittances of thefluorescent materials doped with Ti at 0.1%, 0.3% and 1% in terms ofmolar ratio to Al₂, respectively.

From FIG. 7, it is seen that the matrix crystal has no absorption peakin a wavelength range of 200 nm to 900 nm except for the band edgeabsorption and has property needed for an optical crystal. It is alsoseen that a fluorescent material doped with Ti has no absorption peak ina wavelength range of 200 nm to 900 nm except for the band edgeabsorption. It is further seen that the transmittance rises as the Tidoping amount is increased.

FIG. 8 is a graph showing spectra of light emissions by band edgeexcitation light of Ti doped fluorescent materials according to thepresent invention. In the graph, the abscissa axis represents theemission wavelength and the ordinate axis represents the emissionintensity in an arbitrary scale. Curve (1), curve (2), curve (3) andcurve (4) indicate the emission spectra of the fluorescent materialsdoped with Ti at 0.3%, 0.5%, 1% and 0.1% in terms of molar ratio to Al₂,respectively. The band edge excitation light used was an ultraviolet rayof a wavelength around 275 nm where the emission efficiency is thehighest. Note here that the flattened peak of the solid curve (1) is dueto a limit of measurement by the measuring device.

From this graph it is seen that these fluorescent materials arefluorescent materials which by the band edge excitation produce lightemissions having a peak at 490 nm and a full width at half minimum ofabout 130 nm and that their fluorescence intensity becomes the highestwhen the Ti concentration ranges between 0.3% and 1%.

FIG. 9 is a graph showing time resolved spectra of a light emissionspectrum having a peak at 490 nm of a Ti doped fluorescent material. Inthe graph, the abscissa axis represents the emission spectral wavelengthand the ordinate axis represents the emission intensity in an arbitraryscale. A fluorescent material doped with Ti at 0.3% was irradiated withultrashort light pulses having a center wavelength of 275 nm to measurechanges of the 490 nm emission spectrum with time. Respective spectra atsuccessive delays of time after the irradiation are shown, which aredesignated by different type lines, and these delays of time areindicated in the graph.

From this graph, it is seen that the emission spectrum has a timeconstant of about 9 microseconds. And, from the fact that the timeconstant is in the order of microseconds, it is seen that the emissionmechanism is not due to the d-d transition by d electron of Ti, becausethe d-d transition that is an electric dipole forbidden transitionrequires time constant to be in the order of milliseconds. Also, fromthe ESR (electron spin resonance) measurement which has confirmed thatTi of this fluorescent material exists as Ti⁴⁺ at a B site in the spineltype MgAl₂O₄ crystal, it is inferred that the light emission occurs whenthe electron of an electron-hole pair generated by band edge excitationis captured by a Ti⁴⁺ to form an intermediate energy state andrecombines with a hole captured by an O in the vicinity of the Ti⁴⁺. Thelight emission may also occur conversely when a hole is captured by an Oin the vicinity of a Ti⁴⁺ to form an intermediate energy state andrecombines with an electron in the vicinity of the Ti⁴⁺. Since thisemission mechanism is not by electric dipole forbidden transition but bycharge transfer transition which is one or two order higher intransition probability than d-d transition in the electric dipoleforbidden transition, it is seen that a fluorescent material is providedhere having a high emission efficiency rendering it capable of laseroscillation.

EXAMPLE 3

Several source material rods were prepared from mixed source materialscontaining an aluminum oxide (Al₂O₃) material and an magnesium oxide(MgO) material in a proportion such that an amount of MgO exceeds thatof Al₂O₃ by 1% in terms of molar ratio with various amounts of MnO₂ asthe transition metal material added thereto, and the source materialrods were single-crystallized in an Ar or O₂ gas by the floating zonemelting method to form fluorescent materials, and their properties weremeasured.

FIG. 10 is a graph showing transmittance characteristics of Mn dopedfluorescent materials according to the present invention. In the graph,the abscissa and ordinate axes represent the light wavelength andtransmittance, respectively. The dashed dotted line indicates thetransmittance of the matrix crystal while the dotted line, the dashedline and the solid line indicate the transmittances of the fluorescentmaterials doped with Mn at 0.3%, 0.5% and 1% in terms of molar ratio toMg, respectively. From the graph, it is seen that the fluorescentmaterials doped with Mn at 0.3% or less has no absorption peak over awavelength range of 200 nm to 900 nm except for the band edgeabsorption. It is further seen that the transmittance rises as the Mndoping amount is increased, the absorption increases at around 450 nmand the transmittance rises in a wavelength range of 500 nm or longer.This phenomenon is presumably due to the fact that a portion of Mg sitesin the spinel type MgAl₂O₄ crystal is filled with Mn.

FIG. 11 is a graph showing spectra of light emissions by 450 nmexcitation light of Mn doped fluorescent materials according to thepresent invention. In the graph, the abscissa axis represents the lightwavelength and the ordinate axis represents the light intensity. Thedotted line, the dashed line and the solid line indicate the emissionspectra of the fluorescent materials doped with Mn at 0.3%, 0.5% and 1%in terms of molar ratio to Mg, respectively. From the graph, it is seenthat the 450 nm excitation produces light emissions having a peak at 520nm and a full width at half maximum of 30 nm. Note here that though notshown, it has been confirmed that an emission peak at 520 nm is producedby band edge excitation, too, with a quantum efficiency of the samedegree.

FIG. 12 is a graph showing excitation spectra of emission spectra havinga peak at 520 nm of Mn doped fluorescent materials according to thepresent invention. In the graph, the abscissa axis represents thewavelength of excitation light and the ordinate axis represents theintensity of excitation light in an arbitrary scale. The dotted line,the dashed line and the solid line indicate the excitation spectra offluorescent materials doped with Mn at 0.3%, 0.5% and 1% by molar ratioto Mg. From the graph it is seen that there exist two excitation peaksat around 450 nm and two excitation peaks at 400 nm or less.

FIG. 13 is a graph showing time resolved spectra of an emission spectrumhaving a peak at 520 nm of a Mn doped fluorescent material according tothe present invention. The abscissa axis represents the wavelength of anemission spectrum and the ordinate axis represents the emissionintensity in an arbitrary scale. The fluorescent material doped with Mnat 0.3% was irradiated with ultrashort light pulses having a centralwavelength of 520 nm, and changes of the emission spectrum of 520 nmwith time were measured. Respective spectra at successive delays of timeafter the irradiation are shown, which are designated by different linetypes, and these delays of time are indicated in the graph.

From this graph, it is seen that the emission spectrum has a timeconstant as long as about 1 millisecond. From the four excitationspectra in FIG. 12 and the time constant from FIG. 13, it is seen thatthe emission mechanism is based on d-d transitions in the ligand fieldof 4 oxygens, by d electrons of Mn²⁺ ions positioned at A sites in thespinel type crystal. Since most of d-d transitions are electric dipoleforbidden transitions, the time constant is long. Though not shown, ithas also been confirmed that the emission when by band edge excitationhas a time constant in the order of microseconds, indicating that chargetransfer transitions are brought about in this case.

FIG. 14 is a graph showing spectra of light emissions by band edgeexcitation of Mn doped fluorescent materials according to the presentinvention. In the graph, the abscissa axis represents the emissionwavelength and the ordinate axis represents the emission intensity in anarbitrary scale. In the graph, curve (1), curve (2) and curve (3)indicate the emission spectra of the fluorescent materials doped with Mnat 0.3%, 0.5% and 1% in terms of molar ratio to Mg, respectively. Theband edge excitation light source used was a source of an ultravioletray of a wavelength around 260 nm where the emission efficiency is thehighest. From the graph, it is seen that these fluorescent materialsproduce by the band edge excitation light emissions whose spectra havinga peak at 650 nm and a full width at half maximum of about 70 nm andtheir fluorescence intensity is the highest when the Mn concentrationranges between 0.3% and 1%.

FIG. 15 is a graph showing time resolved spectra of an emission spectrumhaving a peak at 650 nm of a Mn doped fluorescent material according tothe present invention. The abscissa axis represents the emissionspectral wavelength and the ordinate axis represents the emissionintensity in an arbitrary scale. The fluorescent material doped with Mnat 0.3% was irradiated with ultrashort light pulses having a wavelengthof 260 nm, and changes of the emission spectrum of 650 nm with time weremeasured. Respective spectra at successive delays of time after theirradiation are shown and identified with different types of lines withindications of these delays of time in the graph for the spectra. Fromthis graph, it is seen that the emission spectrum having a peak at 650nm has a time constant in the order of several tens microseconds.

From this attenuation constant, it is inferred that the emission at 650nm occurs by Mn existing as Mn²⁺ at an A site in the spinel type MgAl₂O₄crystal, the electron of an electron-hole pair generated by band edgeexcitation is captured by a Mn²⁺ to form an intermediate energy stateand recombines with a hole captured by an O in the vicinity of the Mn²⁺.Conversely, it is inferred that it occurs so that a hole is captured byan O in the vicinity of a Mn²⁺ to form an intermediate energy state andrecombines with an electron in the vicinity of the Mn²⁺. Since thisemission process is not by electric dipole forbidden transition and itis one or two order higher in transition probability than d-d transitionin the electric dipole forbidden transition, it is seen that afluorescent material is provided here having a high emission efficiencyrendering it capable of laser oscillation.

EXAMPLE 4

Several source material rods were prepared from mixed source materialscontaining an aluminum oxide (Al₂O₃) material and a magnesium oxide(MgO) material in a proportion such that an amount of MgO exceeds thatof Al₂O₃ by 1% in terms of molar ratio with various amounts of a V metalas the transition metal material added thereto and the source materialrods were single-crystallized in an Ar or O₂ gas by the floating zonemelting method to form fluorescent materials, and their properties weremeasured.

FIG. 16 is a graph showing emission spectra of light emissions by bandedge excitation of V doped fluorescent materials according to thepresent invention. In the graph, the abscissa axis represents thefluorescence wavelength and the ordinate axis represents thefluorescence intensity in an arbitrary scale. In the graph, curve (1),curve (2), curve (3) and curve (4) indicate the emission spectra of thefluorescent materials doped with V at 0.1%, 0.3%, 1% and 3% in terms ofmolar ratio to Al₂, respectively. The excitation source used was asource of an ultraviolet ray of a wavelength around 330 nm where theemission efficiency is the highest. From the graph, it is seen thatthese fluorescent materials produce white fluorescent emissions over awavelength range from 450 nm to 750 nm and that their fluorescenceintensity is the highest when the V concentration ranges between 0.1%and 1%.

EXAMPLE 5

Using as a target a sintered body of a mixed raw material containing Mg,Al, Ti and O in a chemical equivalent ratio according to compositionformula: MgAl_(2-x)Ti_(x)O₄ where 0.003≦x≦0.01, a thin film singlecrystal fluorescent material was epitaxially grown on a SrTiO₃ (100)substrate by laser ablation in an O₂ gas and its properties weremeasured.

FIG. 17 is a picture showing a RHEED (reflection high-energy electrondiffraction) pattern of the thin film single crystal fluorescentmaterial of MgAl_(2-x)Ti_(x)O₄ grown epitaxially on the SrTiO₃ (100)substrate. From this diffraction pattern, it is seen that the thin filmsingle crystal fluorescent material of the present invention is a singlecrystalline thin film.

FIG. 18 is a chart showing an X-ray diffraction pattern of the thin filmsingle crystal fluorescent material of MgAl_(2-x)Ti_(x)O₄. From thechart it is seen that the thin film formed here is a Ti doped spineltype MgAl₂O₄ single crystalline thin film.

FIG. 19 is a photograph showing fluorescence emitted from the thin filmsingle crystal fluorescent material of MgAl_(2-x)Ti_(x)O₄ when it isexcited by an electron beam energized with an accelerating voltage of 20keV and applied from the rear surface of the substrate. From thepicture, it is seen that strong blue emission is obtained.

Mention is next made of a laser apparatus having its laser medium formedof a transition metal doped spinel type MgAl₂O₄ fluorescent materialaccording to the present invention.

The laser apparatus in accordance with the present invention has itslaser medium made of a Ti, Mn, or V doped spinel type MgAl₂O₄fluorescent material according to the present invention as mentionedabove.

FIG. 20 shows embodiments in setup of the laser apparatus according tothe present invention. As shown in FIG. 20(a) a laser apparatus 60 ofthe present invention includes a resonator comprising a servo prism 61and a semi-transparent mirror 62 and a laser medium 63 disposed in theresonator and made of a spinel type MgAl₂O₄ fluorescent material dopedwith Ti, Mn or V whereby irradiating the laser medium laterally withexcitation light 64 produces resonant light 65 and causes laseroscillation light 66 to be emitted.

As shown in FIG. 20(b) a laser apparatus 70 of the present invention hasthe laser medium 63 of spinel type MgAl₂O₄ fluorescent material dopedwith Ti, Mn or V disposed in a resonator made of the prism 61 and aprism 67 and includes a broadband prism 68 laterally coupled to theprism 67 whereby irradiating the laser medium 63 laterally withexcitation light 64 produces resonant lights 65 and causes laseroscillation light 66 to be emitted laterally from the broadband prism68. Here, the bottom surface of the broadband prism 68 has a reflectionfilm 68 a of 100% reflectance.

Further, as shown in FIG. 20(c) a laser apparatus 80 of the presentinvention has laser media 63 a and 63 b made of Ti doped spinel typeMgAl₂O₄ fluorescent material and a laser medium 63 c made of Mn dopedspinel type MgAl₂O₄ fluorescent material disposed in series in theresonator made of the prism 61 and the semi-transparent mirror 62whereby irradiating the laser media 63 a, 63 b and 63 c laterally withexcitation light 64 produces resonant lights 65 and causes laseroscillation lights 66 a, 66 b and 66 c to be emitted.

For the excitation light 64, use may be made of the fourth harmonic(having a wavelength of 266 nm) of a Nd: YAG laser, the fourth harmonic(having a wavelength of 262 nm) of a Nd: YLF or the fourth harmonic(having a wavelength of 269 nm) of a Nd: YAP laser, as its source.

In the embodiments shown in FIGS. 20(a) and (b), the use of a Ti dopedspinel type MgAl₂O₄ fluorescent material for the laser medium allowsblue or green colored laser oscillation light to be obtained byutilizing light emission having a peak at 490 nm of this fluorescentmaterial. Also, the use of a Mn doped spinel type MgAl₂O₄ fluorescentmaterial for the laser medium allows red colored laser oscillation lightto be obtained by utilizing light emission having a peak at 650 nm ofthis fluorescent material. Also, the use of a V doped spinel typeMgAl₂O₄ fluorescent material for the laser medium allows white coloredlaser oscillation light to be obtained by utilizing broad lightemissions from 400 nm to 800 nm of this fluorescent material.

According to the embodiment shown in FIG. 20(c), blue and green coloredlaser oscillation light emissions can be obtained respectively from thetwo laser media of Ti doped spinel type MgAl₂O₄ fluorescent material 63a and 63 b and red colored laser light emission from the laser medium ofMn doped spinel type MgAl₂O₄ fluorescent material 63 c, concurrently;hence there result laser light emissions of the three primary colors.

Also, in another embodiment not shown in diagram, a laser medium of Tidoped spinel type MgAl₂O₄ fluorescent material and two laser media of Mndoped spinel type MgAl₂O₄ fluorescent material may be disposed in seriesin an external laser resonator and these three laser media may beside-pumped with the fourth harmonic (having a wavelength of 266 nm) ofa Nd: YAG laser, the fourth harmonic (having a wavelength of 262 nm) ofa Nd: YLF, or the fourth harmonic (having a wavelength of 269 nm) of aNd: YAP laser, making it possible to produce blue or green emission fromthe laser medium of Ti doped spinel type MgAl₂O₄ fluorescent material,and green or blue and red emissions, respectively, from the two lasermedia of Mn doped spinel type MgAl₂O₄ fluorescent material,simultaneously, and thus to obtain a laser of the three primary colors.

Also, in another embodiment not shown in diagram, a first laser mediumof Ti doped spinel type MgAl₂O₄ fluorescent material and a second lasermedium of Mn doped spinel type MgAl₂O₄ fluorescent material may bedisposed in series in an external laser resonator and these two lasermedia of fluorescent materials may be side-pumped with the fourthharmonic (having a wavelength of 266 nm) of a Nd: YAG laser, the fourthharmonic (having a wavelength of 262 nm) of a Nd: YLF, or the fourthharmonic (having a wavelength of 269 nm) of a Nd: YAP laser, making itpossible to produce blue or green emission from the Ti doped spinel typeMgAl₂O₄ fluorescent material, and green or blue and red emissions fromthe Mn doped spinel type MgAl₂O₄ fluorescent material, simultaneously,and thus to obtain a laser of the three primary colors.

Mention is next made of a specific example of the laser apparatusaccording to the present invention. In the makeup shown in FIG. 20(a),use was made of a Ti doped spinel type MgAl₂O₄ fluorescent material anda Mn doped spinel type MgAl₂O₄ fluorescent material for the laser mediumand of a Nd: YAG (266 nm) for the excitation light source to producelaser oscillations. The prism was composed of fused quartz, the mirrorhad a reflectance of 98%, the laser medium had a length of about 2 cmand the excitation light had an intensity of 10 MW/cm².

FIG. 21 is a photograph showing oscillations of such a Ti doped spineltype MgAl₂O₄ fluorescent material laser apparatus of the presentinvention, in which FIG. 21 (a) shows blue color laser oscillationsobtained using a Ti doped spinel type MgAl₂O₄ fluorescent material asthe laser medium and FIG. 21(b) is a near-field pattern of the bluecolor oscillations shown in FIG. 21(a).

FIG. 22 shows orange (red) color laser oscillations of such a Mn dopedspinel type MgAl₂O₄ fluorescent material laser apparatus of the presentinvention.

Each of these photographs was taken at a distance of about 1 m obliquelyfrom a screen irradiated by laser light as seen in its center of thephotographs. Due to high intensity of the laser light, the entirelaboratory seems colored with the laser oscillation light.

INDUSTRIAL APPLICABILITY

High in crystallographic perfection of its matrix crystal and also highin its fluorescent emission efficiency, a transition metal doped spineltype MgAl₂O₄ fluorescent material according to the present invention canbe used as a laser medium of a laser of external resonator type and canbe used in a laser oscillation apparatus for emissions with variouswavelengths in a visible light range. Also, when used as a laser mediumin a light source of ultrashort light pulses, it can be used in anultrashort light pulse laser apparatus having a wavelength center in avisible light region. Further, it is useful to use it as a fluorescentmaterial for many kind of color displays as well.

1. A transition metal doped spinel type MgAl₂O₄ fluorescent material,characterized in that it is made by single-crystallizing a sourcematerial rod in a selected gaseous atmosphere, wherein the sourcematerial rod is formed from a mixed raw material of an Al raw materialand a Mg raw material so that the amount of Mg exceeds that of Al by afew percents in terms of molar ratio, added thereto together in anamount with a transition metal raw material.
 2. A transition metal dopedspinel type MgAl₂O₄ fluorescent material as set forth in claim 1,characterized in that said source material rod is formed by forming saidmixed raw material under pressure and is single-crystallized by floatingzone melting.
 3. A transition metal doped spinel type MgAl₂O₄fluorescent material as set forth in claim 1, characterized in that saidgaseous atmosphere is an oxidizing or inert gas.
 4. A transition metaldoped spinel type MgAl₂O₄ fluorescent material as set forth in claim 1,characterized in that said transition metal is Ti and said transitionmetal doped spinel type MgAl₂O₄ fluorescent material has a compositionexpressed by chemical formula MgAl_(2-x)Ti_(x)O₄ where 0.003≦x≦0.01, andthat this fluorescent material has no absorption except for its bandedge absorption in a light wavelength range of 200 nm to 900 nm and itis capable of light emission having a peak at 490 nm by its band edgeexcitation.
 5. A transition metal doped spinel type MgAl₂O₄ fluorescentmaterial as set forth in claim 1, characterized in that said transitionmetal is Mn and said transition metal doped spinel type MgAl₂O₄fluorescent material has a composition expressed by chemical formulaMg_(1-x)Mn_(x)AlO₄ where 0.003≦x≦0.01, and that this fluorescentmaterial has its band edge absorption and absorption having anabsorption peak at 450 nm which increases proportionally as the amountof Mn doped is increased in a light wavelength range of 200 nm to 900 nmand it is capable of light emission having a peak at 520 nm byexcitation of 450 nm light and having a peak at 650 nm by its band edgeexcitation.
 6. A transition metal doped spinel type MgAl₂O₄ fluorescentmaterial as set forth in claim 1, characterized in that said transitionmetal is V and said transition metal doped spinel type MgAl₂O₄fluorescent material has a composition expressed by chemical formulaMgAl_(2-x)V_(x)O₄ where 0.001≦x≦0.01, and that this fluorescent materialhas no absorption peak except for its band edge absorption in a lightwavelength range of 200 nm to 900 nm and it is capable of white colorlight emission over wavelengths of 450 nm to 750 nm by its band edgeexcitation.
 7. A laser apparatus having a laser medium made of atransition metal doped spinel type MgAl₂O₄ fluorescent material as setforth in any one of claims 1 to
 6. 8. A laser apparatus having a lasermedium made of a transition metal doped spinel type MgAl₂O₄ fluorescentmaterial, characterized in that the laser medium is made of afluorescent material as set forth in claim 4 and that said laser mediumis side-pumped with a fourth harmonic of a Nd: YAG laser (having awavelength of 266 nm), a fourth harmonic of a Nd: YLF (having awavelength of 262 nm), or a fourth harmonic of a Nd: YAP laser (having awavelength of 269 nm) and is caused to resonate with an external laserresonator to produce blue or green color laser light utilizing lightemission having the peak at 490 nm of said fluorescent material.
 9. Alaser apparatus having a laser medium made of a transition metal dopedspinel type MgAl₂O₄ fluorescent material, characterized in that thelaser medium is made of a fluorescent material as set forth in claim 5and that said laser medium is side-pumped with a fourth harmonic of aNd: YAG laser (having a wavelength of 266 nm), a fourth harmonic of aNd: YLF (having a wavelength of 262 nm), or a fourth harmonic of a Nd:YAP laser (having a wavelength of 269 nm) and is caused to resonate withan external laser resonator to produce red color laser light utilizinglight emission having the peak at 650 nm of said fluorescent material.10. A laser apparatus having a laser medium made of a transition metaldoped spinel type MgAl₂O₄ fluorescent material, characterized in thattwo laser media made of a fluorescent material as set forth in claim 1,characterized in that said transition metal is Ti and said transitionmetal doped spinel type MgAl₂O₄ fluorescent material has a compositionexpressed by chemical formula MgAl_(2-x)Ti_(x)O₄ where 0.003≦x≦0.01, andthat this fluorescent material has no absorption except for its bandedge absorption in a light wavelength range of 200 nm to 900 nm and itis capable of light emission having a peak at 490 nm by its band edgeexcitation and a laser medium made of a fluorescent materialcharacterized in that said transition metal is Mn and said transitionmetal doped spinel type MgAl₂O₄ fluorescent material has a compositionexpressed by chemical formula Mg_(1-x)Mn_(x)AlO₄ where 0.003≦x≦0.01 andthat this fluorescent material has its band edge absorption andabsorption having an absorption peak at 450 nm which increasesproportionally as the amount of Mn doped is increased in a lightwavelength range of 200 nm to 900 nm and it is capable of light emissionhaving a peak at 520 nm by excitation of 450 nm light and having a peakat 650 nm by its band edge excitation are disposed in series in anexternal laser resonator and these three laser media are side-pumpedwith a fourth harmonic of a Nd: YAG laser (having a wavelength of 266nm), a fourth harmonic of a Nd: YLF (having a wavelength of 262 nm), ora fourth harmonic of a Nd: YAP laser (having a wavelength of 269 nm) toproduce blue and green color laser oscillations, respectively, from thefirst two laser media and red color laser oscillation from the thirdlaser medium, simultaneously.
 11. A laser apparatus having a lasermedium made of a transition metal doped spinel type MgAl₂O₄ fluorescentmaterial, characterized in that a laser media made of a fluorescentmaterial as set forth in claim 1, characterized in that said transitionmetal is Ti and said transition metal doped spinel type MgAl₂O₄fluorescent material has a composition expressed by chemical formulaMgAl_(2-x)Ti_(x)O₄ where 0.003≦x≦0.01, and that this fluorescentmaterial has no absorption except for its band edge absorption in alight wavelength range of 200 nm to 900 nm and it is capable of lightemission having a peak at 490 nm by its band edge excitation and twolaser media made of a fluorescent material characterized in that saidtransition metal is Mn and said transition metal doped spinel typeMgAl₂O₄ fluorescent material has a composition expressed by chemicalformula Mg_(1-x)Mn_(x)AlO₄ where 0.003≦x≦0.01, and that this fluorescentmaterial has its band edge absorption and absorption having anabsorption peak at 450 nm which increases proportionally as the amountof Mn doped is increased in a light wavelength range of 200 nm to 900 nmand it is capable of light emission having a peak at 520 nm byexcitation of 450 nm light and having a peak at 650 nm by its band edgeexcitation are disposed in series in an external laser resonator andthese three laser media are side-pumped with a fourth harmonic of a Nd:YAG laser (having a wavelength of 266 nm), a fourth harmonic of a Nd:YLF (having a wavelength of 262 nm), or a fourth harmonic of a Nd: YAPlaser (having a wavelength of 269 nm) to produce blue or green colorlaser oscillation from the first laser medium and blue or green and redcolor laser oscillations, respectively, from the second and third lasermedia, simultaneously.
 12. A laser apparatus having a laser medium madeof a transition metal doped spinel type MgAl₂O₄ fluorescent material,characterized in that a first laser media made of a fluorescent materialas set forth in claim 1, characterized in that said transition metal isTi and said transition metal doped spinel type MgAl₂O₄ fluorescentmaterial has a composition expressed by chemical formulaMgAl_(2-x)Ti_(x)O₄ where 0.003≦x≦0.01, and that this fluorescentmaterial has no absorption except for its band edge absorption in alight wavelength range of 200 nm to 900 nm and it is capable of lightemission having a peak at 490 nm by its band edge excitation and asecond laser medium made of a fluorescent material characterized in thatsaid transition metal is Mn and said transition metal doped spinel typeMgAl₂O₄ fluorescent material has a composition expressed by chemicalformula Mg_(1-x)Mn_(x)AlO₄ where 0.003≦x≦0.01, and that this fluorescentmaterial has its band edge absorption and absorption having anabsorption peak at 450 nm which increases proportionally as the amountof Mn doped is increased in a light wavelength range of 200 nm to 900 nmand it is capable of light emission having a peak at 520 nm byexcitation of 450 nm light and having a peak at 650 nm by its band edgeexcitation are disposed in series in an external laser resonator andthese two laser media are side-pumped with a fourth harmonic of a Nd:YAG laser (having a wavelength of 266 nm), a fourth harmonic of a Nd:YLF (having a wavelength of 262 nm), or a fourth harmonic of a Nd: YAPlaser (having a wavelength of 269 nm) to produce blue or green colorlaser oscillation from said first laser medium and blue or green and redcolor laser oscillations from said second laser medium, simultaneously.13. A method of making a transition metal doped spinel type MgAl₂O₄fluorescent material, characterized in that it comprises: preparing amixed raw material of an Al raw material and a Mg raw material so thatthe amount of Mg exceeds that of Al by a few percents in terms of molarratio, adding thereto together in an amount with a transition metal rawmaterial, shaping the mixed raw material under pressure to form a sourcematerial rod, and single-crystallizing the source material rod in aselected gaseous atmosphere by floating zone melting.
 14. A method ofmaking a transition metal doped spinel type MgAl₂O₄ fluorescent materialas set forth in claim 13, characterized in that said selected gaseousatmosphere is an oxidizing or inert gas.
 15. A method of making atransition metal doped spinel type MgAl₂O₄ fluorescent material as setforth in claim 13, characterized in that said Al raw material is Al₂O₃,said Mg raw material is MgO, and said transition metal raw material isTiO₂ and that the raw material TiO₂ is added to the raw material Al₂O₃at a proportion in a range of 0.003 to 0.01 in terms of molar ratio. 16.A method of making a transition metal doped spinel type MgAl₂O₄fluorescent material as set forth in claim 13, characterized in thatsaid Al raw material is Al₂O₃, said Mg raw material is MgO, and saidtransition metal raw material is MnO₂ and that the raw material MnO₂ isadded to the raw material MgO at a proportion in a range of 0.003 to0.01 in terms of molar ratio.
 17. A method of making a transition metaldoped spinel type MgAl₂O₄ fluorescent material as set forth in claim 13,characterized in that said Al raw material is Al₂O₃, said Mg rawmaterial is MgO, and said transition metal raw material is a V metal rawmaterial, and that the V metal raw material is added to the raw materialAl₂O₃ at a proportion in a range of 0.001 to 0.01 in terms of molarratio.
 18. A method of making a transition metal doped spinel typeMgAl₂O₄ fluorescent material, characterized in that it comprises: mixingtogether a source material Al₂O₃, a source material MgO, and atransition metal raw material to form a mixture thereof; shaping underpressure, and sintering the mixture to form a sintered body, andepitaxially growing on a single-crystal substrate a single-crystal thinfilm of transition metal doped spinel type MgAl₂O₄ fluorescent materialby laser ablation using said sintered body as a target in an O₂ gas.